Dispersion-modified cobalt and/or nickel alloy containing anisodiametric grains



Feb. 13, 1968 w. J BARNETT ETAL.

DISPERSION-MODIFIED COBALT AND/OR NICKEL ALL CONTAINING ANISODIAMETRIC GRAINS Filed July 29, 1965 FiG. 2

FIG. 1

INVENTORS WILLIAM J. BARNETT GEROLD A. MANCINI JOHN svmouos gzd 6W ATTORNEY United States Patent 3,368,883 DISPERSION-MODIFIED COBALT AND/0R NICKEL ALLOY CONTAINING ANISODIA- METRIC GRAINS William .I. Barnett, Brandywine Hundred, DeL, Gerold A. Mancini, Rochester, N.Y., and John Symonds, Annapolis, Md., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed July 29, 1965, Ser. No. 475,670 8 Claims. (Cl. 29182.5)

ABSTRACT OF THE DISCLOSURE Alloy having as metal components, by weight, to 40% of chromium and/or molybdenum and/or tungsten, the maximum of chromium being 35% and of molybdenum plus tungsten 22%, the balance of the metal components being cobalt and/or nickel in the proportion of at least 60%, the alloy being dispersion-modified with up to 6% by volume of refractory metal oxide par ticles having an average size less than 100 millimicrons, and being characterized by containing a substantial proportion of anisodiametric grains with a length-to-thickness ratio greater than 3.

This invention relates to novel processes for producing alloys of nickel and chromium in whichthere is dispersed .a particulate refractory metal oxide and to the products so produced. More particularly the invention is directed "to such processes comprising the steps of forming by powder. metallurgy a consolidated body having the chemical composition desired in the final alloy and containing the refractory oxide particles dispersed in the metal components, working said consolidated body at a temperature of up to 1800 F. to densify it substantially to 100 percent density and to store energy therein sufficient to permit subsequent recrystallization, heat-treating said worked body by heating it to a temperature in the range of 1900 to 2500 F., and working the body to at least 50 percent reduction in cross section area at a temperature in the range from 1300 to 1800" F. The invention is further particularly directed to novel alloy compositions which can be prepared by said processes and which are dispersion-modified alloys comprising, as metallic components, about from 10 to 40 percent of at least one alloying metal selected from the group consisting of chromium, molybdenum and tungsten, the maximum concentration of chromium being 35 percent, and the maximum concentration of molybdenum plus tungsten being 22 percent, and at least 60 percent of cobalt, nickel, or a mixture of cobalt and nickel, there being pervasively dispersed in said metallic components up to 6 percent by volume of refractory metal oxide particles having an average size less than 100 millimicrons, said metal oxide having a free energy of formation at 1000 C. greater than 106 kilocalories per gram atom of oxygen, and said alloy, upon metallographic examination, showing a substantial proportion of anisodiametric grains having a length-to-thickness ratio greater than 3 and preferably greater than 10.

In the drawings:

FIGURE 1 is a line drawing of a photomicrograph at 500 times magnification of the product of Example 1, and

FIGURE 2 is a line drawing of a photomicrograph at 100 times magnification of the product of Example 2.

The lines in both of these figures represent grain boundaries, although some of the lines in FIGURE 1 are twin boundaries.

In United States Patent 2,972,529, there has been described certain novel compositions and processes for ICC making them, the compositions comprising a chromium alloy of iron, cobalt or nickel dispersion-hardened with a particulate refractory metal oxide of high free energy of formation. This patent describes the preparation cf these compositions in powder form and the consolidation of such powders to useful solid metal bodies by such processes as hot extrusion, swaging or forging. The products obtained are described as fine grained, the grain size being less than 10 microns, and the grains are substantially isodiametricthat is, being substantially equiaxial. These are strong, useful compositions and alloys. It has now been found, however, that by the processes of the present invention stronger, more creep-resistant, more ductile products can be produced than those described in the prior patent.

In US. Patent 3,159,908, there has been described processes for working oxide-dispersion-modified metals at low temperatures, i.e., below half of the absolute melting point of the metal, which in the case of nickel base alloys is below about 1100 F.

It has now been found that, in the case of chromiumcontaining alloys, highest strength properties are obtained when the dispersion-modified alloy has undergone a particular heat treating and working sequence, namely, the dispersion-modified alloy powder is consolidated in such a manner as to leave stored energy in the body, and thereafter, this material is heated to the range of 2100 to 2500 F. and worked to at least 50 percent reduction in the temperature range of 1300 to 2200 F.

The powder used in the processes of the invention is broadly described in US. 2,972,529. Preferred powders are prepared as follows: nickel oxide-chromium oxidethori-a is prepared from the respective nitrates by co precipitation with (NH CO in aqueous solutions. The oxide is dried at 300 C., blended with low sulfur carbon, parts oxide to 9 to 12 parts carbon, and treated with hydrogen at 400 C. to reduce the NiO and with a mixture of hydrogen-methane at 900950 C. to reduce the chromium oxide. The excess carbon is then removed with pure hydrogen under pressure at a temperature in the range 750 to 900 C.

The resulting powder is consolidated by powder metallurgy, for example by compacting, 'sintering in hydrogen at 900-1000 C., and extruding to a bar at about 1000" C. This consolidates the powder to a metallurgical workpiece of greater than 99% of theoretical density.

The solid metal product is heated until it recrystallizes. As-extruded the has consists of a fine grained material. On heating to a temperature in the range of 1900 to 2500 F., the grains grow, the resulting grains being anisodiametric -and elongated in the direction of working.

The product after extrusion is also warm-worked. The term work generically is defined as being a process for reducing bar diameters, such as bar-rolling, swaging, drawing, forging, and other similar, conventional metallurgical processes. Warm-working temperatures are in the range of 1300 to 1800 F., work can be to at least 50 percent reduction in area.

It will be understood that the above-described recrystallization heating and warm-working steps can be carried out in either order. In one specific aspectof the invention the consolidated piece is warm-worked and then heat treated. In this sequence, the optimum working temperature is about 1500 F., and must be in the range 1300 to 1800 F. In another specific, preferred aspect, the consolidated, warm-worked piece is heat-treated and thereafter hot-worked. In this case the hot-working temperature should be in the range 1800-2200 F. Working can be done in air, as can heat-treating. The material is so oxidation-resistant that scale-formation is negligible. In a further specific aspect the consolidated piece is first heat-treated to effect recrystallization and is thereafter warm-worked.

The products of invention are essentially nickel or cobalt base alloys, containing at least 60 percent by weight of these elements. They contain about from to 40 percent of at least one alloying metal selected from the group consisting of chromium, molybdenum and tungsten, the maximum concentration of chromium being 35 percent, and the maximum concentration of molybdenum plus tungsten being 22 percent. Said alloys optionally may contain minor amounts of other metals such as up to percent manganese, and up to percent iron. They can also contain up to 1 percent of zirconium, hafnium or magnesium.

Products of the invention contain dispersed refractory oxides, the average diameter of which is less than 100 millimicrons and preferably less than millirnicrons. Such oxides must have a free energy of formation at 1000 C. above 106 kilocalories per gram atom of oxygen in the oxide. Typical of such oxides are Y O CaO, La O ThO BeO, and MgO. The proportion of refractory oxide can be up to 6 percent by volume, 0.1 to 3 percent being a preferred range.

The particle diameter of the refractory oxide filler particles can be calculated from a measurement of their surface area. The metal component of a powder product of the invention is dissolved in an acid, or in brominemethanol, leaving the filler oxide particles, which are recovered by coagulating, centrifuging, washing and drying.

The Br -CH OH extraction procedure is as follows: Calculate the weight of metal for extraction required to give approximately 0.2 gm. ThO residue. Thus, 10 gm. of a metal containing 2 percent ThO- are required. For each 10 gram portion of metal, prepare 500 ml. of solution containing 5.3 percent Br by volume in dry methanol. Subdivide the metal. If dense, machine to chips. Add the metal slowly with stirring to the Br -CH OH solution. Place the solution in a water bath, and cool during the addition. (Temperature should be C.) Avoid frothing caused by excessive gas evolution. After all the metal is added, remove the solution from the water bath, and allow to stand 24 hours with occasional stirring. Allow the residue to settle. Carefully decant the clear supernatant. Centrifuge the remaining residue. Wash and centrifuge the solid residue repeatedly with dry methanol until the supernatant liquor is colorless. Retain all decants and washings for 24 hours to see if additional residue settles out. If so, repeat the centrifuging and washing procedure so as to include this material with the original residue. If, during washing, the ThO residue begins to peptize, floc the material by adding 2 to 3 drops of concentrated HNO then continue centrifuging. Dry the final, washed residue and weigh.

The surface area of the recovered oxide from the above-described process is then measured by the conventional BET method or its equivalent. (P. H. Emmett in Symposium on New Methods for Particle Size Determination in the Subsieve Range, Philadelphia: ASTM, 1941, p. 95.) From this surface area measurement, the mean particle diameter, D, is calculated from the expression:

where f is the absolute density of the filler oxide particles in grams per milliliter and Af is their surface area in square meters per gram.

The grains in the products of invention are anisodiametric. At least 25 percent of the grain structure consists of grains which have a length-to-thickness ratio greater than about 3:1. Preferably up to 60 percent or more of the grains are in this category, i.e., having a length-to-thickness ratio above 3. The length-to-thickness ratio of the grains is greater than 3 and preferably greater than 10. In the most preferred instances the ratio can be up to 25 or even greater. It will be evident that these grains can be in the form of needles or platelets and as observed in the metallographic examination this will depend on whether viewed in the longitudinal or transverse direction. This can be determined by viewing the product in both the longitudinal and transverse direction.

It has been noticed in particular that some of the grains have sharp apexes at the ends-that is the grains seem to be pointed when viewed in a longitudinal direction. This seems to be associated with good strength of the product.

When a product is referred to as strong in this application this refers to the 2000 F. properties of tensile strength and stress rupture. In particular, a stress-rupture life in excess of 20 hours at 6 K s.i. at 2000 F. is strong.

Preferred limits on products of the invention are:

(1) Excess oxygen-less than 1000 p.p.m.most preferred under 200 p.p.m.

(2) Carbon less than p.p.m. Present as (3) Sulfur less than 50 p.p.m. incidental im- (4) Nitrogen less than 200 p.p.m. purities.

The invention will be better understood by reference to the following illustrative examples:

Example 1 300 lb. of Cr(NO -9H O, and 10 lb. of Th(NO -4H O in 420 lb. of water. The solutions were mixed in a tank initially charged with about 6 gallons of liquor from the filtration of a previously prepared gel, the rates of addition being controlled so as to maintain the pH of the solution in the tank at about 7.0 and to complete the additions in 3% hours.

The precipitate was filtered, washed with water, and dried at about C. for 16 hours. The temperature was then increased to 450 C., for 4 hours to remove residual nitrates and carbonate and convert the product entirely to an oxide mixture. The dried product was pulverized in a hammer-mill.

A portion of the oxide product was blended with 12 parts carbon black per 100 parts oxide and the blended mixture was heated to about 425 C. for about 4 hours with hydrogen gas passing over it, the hydrogen having previously been carefully freed of oxygen, sulfur, nitrogen, and moisture. The hydrogen flow was then replaced with argon at a flow rate of about 5 linear feet per minute. The temperature was raised to about 975 C., argon was replaced with hydrogen, and the temperature was held at 975 C. for 20 hours. The temperature was then raised to 1100 C., where it was held for 5 hours after which time the furnace was cooled to 925 C. under hydrogen for 30 hours to remove the excess carbon as methane and then to room temperature. The sintered product was recovered, ground, and passed through a 60-mesh screen.

The product thus obtained was a fine powder containing thoria at a volume loading of 2 percent uniformly dispersed in an alloy matrix of nickel and chomium combined in the ratio of 80 percent Ni-20 percent Cr by weight. Oxygen analyses of the reduced material showed that there was less than 0.05 percent oxygen present in excess of the oxygen in the ThO and the residual carbon analysis was 151 parts per million. Sulfur was 30 p.p.m. ThO size by X-ray line broadening was 50 m and by the BET method was 35 m The Ni-Cr-ThO powder so produced was compacted hydrostatically at 60,000 p.s.i., the compacted billet ma- It was then extruded at 1700 F. at 10:1 ratio to a bar.

Analytical data on this bar were: Nitrogen 215 p.p.m., sulfur 31 p.p.m., carbon 170 p.p.m., chromium 20.4 percent, iron less than 100 p.p.n1., ThO size (by brominemethanol extraction and surface area) 30 millimicrons.

A piece of this bar was heat treated at 2100 F. for one hour, whereupon it recrystallized. The recrystallized bar was swaged at 1800" F. from 1.5 to 1.0 inches diameter. This bar is an example of the products of the invention.

The appearance of the microstructure of this swaged bar remained unchanged after heating to 2400 F. for one hour. X-ray back reflection patterns confirmed that the as-swaged bar was cold-worked and that this coldwork was retained after the heating at 2400F. The 2000 F. strength properties were identical in the asswaged and one hour/2400 F. heat treated pieces. These were: U.T.S.-17,000 p.s.i., 0.2 percent Y.S-16,000 p.s.i., elongation 15 percent, reduction in area 13 percent. Stress rupture at 2000 F. on a bar step-loaded from 6,000 p.s.i. was greater than 90 hours at 11,000 p.s.1.

The microstructure of this bar consisted of a mixture of various types of grains. A striking feature was the presence of many extremely thin needle-like grains typically .025 mm. long by .001 mm.

The indentor test on a longitudinal section from this bar showed a cubic impression, i.e., a square shaped impression with sides parallel and perpendicular to the bar axis. This was identical in as-swaged or in one hour/ 2400 F. heat treated bar.

Example 2 Another bar was made from the extrusion in Example 1 as follows: The 1.5" bar was swaged 1500 F. to 0.5" diameter. This was accomplished in passes with minute reheats between passes. The /2" diameter bar was recrystallized by a one hour/2400 F./ air heat treatment.

Tensile properties of the recrystallized bar at 2000 F. were:

U.T.S. K s.i 140-175 0.2% Y.S K s.i 14.0-17.5 Elongation percent 7 RA. do 3 A 2000 F. stress-rupture test was carried out on the same material; it was loaded at 6 K s.i. (6,000 p.s.i.) for 20 hours, then step-loaded to 16 K s.i. where failure occurred after a total life of 49 hours.

The room temperature hardness of the recrystallized bar was R 24. Microstructural features of this bar were:

(1) Most of-the structure consisted of very elongated grains 0.100.25 mm. wide. L/W ratio of these grains was up to 20. L/W of 10 was common.

(2) The elongated grains tended to have sharper apexes and had less serrated grain-boundaries than in weaker material.

(3) There was preferred orientation in the structure as indicated by the R indentor test. This round indentor gave a square shaped impression in a longitudinal section. The diagonal of the square is oriented parallel to the bar axis.

Example 3 This example is like the previous examples except as hereafter noted:

A rod was prepared from a powder composition like that of Example 1, containing 30 111,11. thoria, by compacting the powder, sintering, extruding 8:1 at 1700 F. and then swaging about 70 percent reduction in area in approximately 12 percent steps heating before each pass to 1600 F.:100" F. The resultant rod was exposed to 2000 F. and recrystallized to coarse grains. The grains were anisodiametric, and about 20 to 200 microns in length.

Example 4 similar to that in Example 1. The chemical composition of the thoriated nickel-chromium-molybdenum alloy powder was, by weight: 4.52% ThO 11.16% Mo, 15.2% Cr, 0.0046% C, balance Ni. Three-tenths percent by weight of zirconium hydride powder having an average particle size less than 10 microns was cone-blended for 2 hours with the minus 200 mesh fraction of the thoriated alloy powder.

A billet was prepared by hydrostatically compacting the resulting powder blend at 60,000 p.s.i. The billet was machined to a right circular cylinder and was welded into a mild steel can containing entrance and exit tubes for passing hydrogen over the billet and for evacuation. The canned billet was evacuated at room temperature to a pressure of less than about 50 microns and back-filled with pure, dry hydrogen. The billet was heated slowly to 580 F. under a flow of about 7 cubic feet per hour of said hydrogen. After about 16 hours at 580 F. the temperature was increased to 680 F., held there for about 1% hours, raised to 930 F. for 1 hour, and finally raised to 1700 F. After about 2 hours at 1700 F., hydrogen flow was terminated, and the canned billet was evacuated at temperature. After about 2 hours at temperature under vacuum an ultimate pressure of about 10 microns was attained.

The canned billet was then cooled to ambient temperature under vacuum, and the exit and entrance tubes were forge-welded shut. The canned billet was extruded at 1700 F. to a reduction ratio of 8/1. After extrusion the mild steel can was removed by pickling.

The thoriated nickel-chromium-molybdenurn alloy bar was then canned in stainless steel tubing, heated to 1500 F., and swaged, with reheating after each pass, to a re duction of 67.8 percent in cross-sectional area. After swaging the stainless steel can was removed by pickling. Hardness of the swaged bar was approximately 484 D.P.H..

The swaged bar was annealed at 2200 F. for 2 hours. Hardness of the annealed bar was about 377 Diamond Pyramid Hardness. Metallographic examination of the annealed bar revealed a relatively uniform, recrytsallized grain structure, a substantial portion of the structure being composed of anisodiametric grains having a length/thickness ratio greater than about 5.

Stress rupture properties of samples cut from the annealed bar were:

A thoriated nickel-19 percent chromium-6 percent molybdenum alloy powder was prepared by a process similar to that used in Example 4. The chemical composition of the thoriated nickel-chromium-molybdenum alloy powder 7 Was, by weight: 4.88% ThO 5.90% M0, 19.34% Cr, balance Ni. Three-tenths percent by weight of zirconium hydride powder having an average particle size less than microns, was cone-blended for 2 hours with the minus 200 mesh fraction of the thoriated alloy powder.

A billet was prepared by hydrostatically compacting the resulting powder blend at 60,000 p.s.i. The billet was machined to a right circular cylinder and was welded into a mild steel can containing entrance and exit tubes for passing hydrogen over the billet and for evacuation. The canned billet was evacuated at room temperature to a pressure of less than about 50 microns and back-filled with pure, dry hydrogen. The billet was heated slowly to 525 F. under a How of about 7 cubic feet per hour of said hydrogen. After about 9 /2 hours at 525 F. the temperature was increased to 700 F., held there for about 2 hours, raised to 815 F. and finally to 1625 F. After 1% hours at 1625 F., hydrogen flow was terminate-d, and the canned billet was evacuated at temperature. After about 8 hours at temperature under vacuum an ultimate pressure of about 75 microns was attained.

The canned billet was then cooled to ambient temperature under vacuum, and the exit and entrance tubes were forge-welded shut. The canned billet was extruded at 1700" F. to a reduction ratio of 8/1. After extrusion the mild steel can was removed by pickling.

The thoriated nickel-chromium-molybdenum alloy bar was then canned in stainless steel tubing, heated to 1500 F. and swaged, with reheating after each pass, to a reduction of 64.3 percent in cross-sectional area. After swaging the stainless steel can was removed by pickling. Hardness of the swaged bar was approximately 451 D.P.H.

The swaged bar was annealed at 2200 F. for 2 hours. Hardness of the annelaed bar was about 380 Diamond Pyramid Hardness. Metallographic examination of the annealed bar revealed a relatively uniform, recrystallized grain structure, a substantial portion of the structure being composed of anisodiametric grains having a length/thickness ratio greater than about 5.

Stress rupture properties of samples cut from the annealed bar were:

1. A dispersion-modified sintered alloy consisting essentially of as metallic components, by weight, about from 10 to 40 percent of at least one alloying metal selected from the group consisting of chromium, molybdenum and tungsten, the maximum concentration of chromium being 35 percent, and the maximum concentration of molybdenum plus tungsten being 22 percent, up to 0 percent manganese, up to 20 percent iron, and up to 1 6 percent of a metal selected from the group consisting of zirconium, hafnium and magnesium, the balance of metallic components being substantially all cobalt, nickel or a mixture of cobalt and nickel, in the proportion of at least percent, there being pervasively dispersed in said metallic components up to 6 percent by volume of refractory metal oxide particles having an average size less than 100 millimicrons, said metal oxide having a free energy of formation of 1000 C. greater than 106 kilocalories per gram atom of oxygen, and said alloy containing a substantial proportion of anisodiametric grains having a length-to-thickness ratio greater than about 3.

2. A dispersion-modified sintered alloy consisting essentially of as metallic components, by weight, about from 10 to 40 percent of at least one alloying metal selected from the group consisting of chromium, molybdenum and tungsten, the maximum concentration of chromium being 35 percent, and the maximum concentration of molybdenum plus tungsten being 22 percent, up to 15 percent manganese, up to 20 percent iron, and up to 1 percent of a metal selected from the group consisting of zirconum, hafnium and magnesium, the balance of metallic components being substantially all cobalt, nickel, or a mixture of cobalt and nickel, in the proportion of at least 60 percent, there being pervasively dispersed in said metallic components up to 6 percent by volume of refractory metal oxide particles having an average size less than 100 millimicrons, said metal oxide having a free energy of formation at 1000 C. greater than 106 kilocalories per gram atom of oxygen, and said alloy containing a substantial proportion of anisodiametric grains having a length-tothickness ratio greater than about 5.

3. A composition of claim 2 in which the metallic components consist essentially of about from 10 to 35 percent by weight of chromium and the balance substantially nickel.

4. A composition of claim 2 in the form of bar in which the metallic components consist essentially of 20 percent by weight of chromium and the balance substantially nickel.

5. A composition of claim 2 in which the length-tothickness ratio is greater than 10.

6. A composition of claim 2 having a recrystallized structure.

7. A composition of claim 2 having a stable, coldworked structure.

8. A composition of claim 2 in which the metallic components consist essentially of, by weight, about 14 to 25 percent of chromium, 5 to 12 percent of molybdenum, 0.1 to 0.4 percent of zirconium and the balance substantially nickel, at least 0.04 percent, based on the total composition of said zirconium being present in solid solu tion and nickel-zirconium type intermetallic compounds.

References Cited UNITED STATES PATENTS 2,972,529 2/ 1961 Alexander.

3,026,200 3/1962 Gregory 206 X 3,087,234 4/1963 Alexander 75170 X 3,152,389 10/1964 Alexander 75206 X 3,166,416 1/1965 Worn 29182.5 X

L. DEWAYNE RUTLEDGE, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

A. J. STEINER. Assistant Examiner. 

